How to combine low sample renders?
The process that I've seen is combining each image in Blender via a Mix node, and using a mix percentage based on [INCORRECT]---- how many images are being combined (in the example he uses 20% since there are 5 images) ----[/INCORRECT]. See the attached screen shot from Kent Trammell's tutorial on CG Cookie.

EDIT: Actually, Ben Simmonds corrected Kent's math on this tutorial. Here's a quote from Ben on the page:


  HI Kent. I think your math was incorrect for those mix nodes. If you
  think about how each image contributes to the final comp, the first
  image node in that chain is contributing 4x more than the 2nd to the
  final image (0.8 vs 0.2). The correct values for the mix nodes in such
  a chain would be as follows:
  
  1st and 2nd: 50% (1st plus 2nd) and 3rd: 33% (1st, 2nd and 3rd) and
  4th: 25% (1st through 4th) and 5th: 20%
  
  If you were mixing the images all at once then the 20% per image value
  would be correct, but the math is slightly different for mixing them
  pairwise.
  
  Alternatively, the simpler option would be to ADD all of the images
  together, and then divide the pixel values of the resulting image by
  5.